Present day communication systems include interface coupling arrangements wherein data signals from a modem are to be coupled to a telephone link. Desirably, the modem is to be coupled through any telephone instrument without direct connection to a telephone line. Unfortunately, the coupling transfer characteristic of a typical telephone microphone is very non-linear, which creates a severe intefacing problem in that the fidelity of signals which can be coupled from a modem to a telephone line through the standard telephone acoustic transducer device is limited. Moreover, the degree of such non-linearity is variable from telephone to telephone, and varies as a function of time for any particular instrument, so that providing a fixed compensation for the telephone instrument will not eliminate the non-linearity.